The present invention relates to the automatic calibration arts. It finds particular application in conjunction with the automatic calibration of x-ray tubes and will be described with particular reference thereto.
Each model and type of x-ray tube conventionally has a published set of filament emission curves or tables. These curve sets or tables commonly take the form of a graph of filament current vs. tube current or mA for each of a plurality of fixed tube voltages or kV. For example, the curve set might include curves for each of three or four tube voltages between 50 kV and 150 kV.
In an x-ray device, the x-ray tube is commonly operated for a selected duration at a selected tube current and voltage combination. This generates a corresponding amount of x-rays of the appropriate energy to penetrate the patient or subject and properly expose photographic film or provide appropriate x-ray flux for other x-ray detection equipment. Generally, the tube voltage across the anode and cathode is readily set. The tube current is controlled by adjusting the current flowing through the cathode filament. Increasing the filament current increases electron emission from the cathode which increases the tube current or electron flow between the cathode and anode. By referring to the filament emission curve set, the filament current required to produce a selected tube current at a selected tube voltage is readily determined.
Heretofore, x-ray equipment has been calibrated with data taken from the filament emission curves. Most commonly, the filament emission curves were used to set the filament current that would be supplied for each combination of x-ray tube currents and voltages that could be selected. To be sure that these were accurate, an initial calibration process was frequently conducted. Either manually or automatically, exposures were taken with each of a plurality of the selected x-ray tube current and voltage parameters. The actual tube current produced was compared with the selected tube current. When the actual and selected tube currents differed, the filament current was adjusted down or up from the value read from the curves as necessary to being the actual and selected tube currents together.
One of the problems with this prior art calibration technique is that it could damage the x-ray tube filament. The filament has a low impedance and operates at a high current. Filament temperature varies generally with power across it, i.e. I.sup.2 R where I is the filament current and R is the filament resistance and filament current varies generally as V/R, where V is the voltage applied across the filament. Even normal manufacturing tolerances of this filament can cause a major change in its resistance, hence its operating temperature and the resultant tube current. For example, typical tolerances for the filament current on the curve table are on the order of .+-.0.15 amps. A variation of 0.15 filament amps can make a difference of plus or minus 300 to 400 mA in the tube current. Particularly when testing the high tube current values, the filament might produce up to 400 mA more than expected. This extra tube current increases the heating of the anode. A tube current increase of the 300 to 400 milliamp range can increase the anode temperature to the melting point or other thermal damage.
The present invention contemplates a new and improved calibration procedure which does not risk damaging the x-ray tube anode.